Study On Electromagnetic Coupling And Tunable Characteristics Of Chiral Metamaterials

The field of chiral metamaterials belongs to subwavelength electromagnetics.There are some interesting phenomena when electromagnetic waves interact with subwavelength structures,such as negative refractive index,circular dichroism and polarization rotation and so on.Not only was changed the geometric parameters of structures,but also the phase-phase material?VO₂?was used to control electromagnetic characteristics including the transmission,the resonance peaks,the states of polarization and so on.In this paper,absorption resonance modes separation of planar chiral metamaterials were studied.Firstly,we designed a U-shaped open-ring resonant structure?U-SRR?,a single-layer metal?Ag?and a double-layer periodic planar chiral metamaterial respectively to study their transmission spectra.It is found that the absorption resonance peaks of metamaterials in near-infrared band?120 THz-470 THz?spilt from unimodal to bimodal,finally to four,respectively.For the four resonance frequencies,two peaks got slight shift to the high frequency direction,while the other new splitting peaks shifted strongly toward the opposite direction in double-layer structure.LC equivalent circuit modes were used to explain the reason of absorption resonance modes separation.We used finite difference-time domain?FDTD?method to verify the correctness of the theoretical model.The distribution of electric,magnetic field and current at the resonant frequency showed the identical conclusion with LC equivalent circuit modes.The position of the absorption resonance peaks??225 THz?and??392.4 THz?are under the influence of the electric coupling in the same planar layers.In addition,the other peaks??176.6 THz?and??319.4 THz?are mainly affected by the magnetic coupling between the different layers.Moreover,it is possible to adjust the position of the separation peaks according to change the interlayer distance?d?.Finally,the chiral metamaterials structures were further designed to study circular dichroism?CD?and optical rotation?OA?.We designed metal/phase-change materials/metal structures,including sandwich phase-change metamaterials?SPCM?and conjugated phase-change metamaterials?CPCM?,to achieve control polarization of electromagnetic waves.The difference of CD and OA in both two type structures between two states of the phase transformation.We focus on the mechanism of CD in CPCM structures.The particle swarm optimization?PSO?was used to optimize the CD in metal/phase-change materials/metal structures.After optimizing,the max CD of CPCM structure soaring to 0.4 at peak B?1180nm?,and it was greater than that?0.1?in SPCM structure.In the infrared band,the phase changing of VO₂ can effectively control the position of CD in SPCM structures.Correspondingly,it only can change amplitude of CD in CPCM structure.Similarly,phase changing both can adjust the peak of polarization rotation angle?in the phase-change metamaterials.For SPCM and CPCM,the?is tunable at 1400nm.In addition,there is a strong?near 720nm,which satisfies the requirements both the transmittance T?50°and??100°.These properties of planar chiral metamaterials can be extensively designed the next generation optoelectronic devices and systems,such as biosensor detections,polarization regulators,optical switches,super reflectors and so on.